1. Field of the Invention
The invention relates to a process for the production of aqueous solutions of alkali organosiliconates and particularly, those whose organic residue is an alkyl residue with 1 to 3 carbon atoms, a vinyl residue and/or a phenyl residue, and whose alkali residue is a sodium or potassium ion.
2. Description of the Prior Art
Alkali methylsiliconate solutions are usually prepared by hydrolyzing methyltrichlorosilane and dissolving the methylsilicic acid formed in an alkaline solution. In this process, it is necessary to filter off the methylsilicic acid and to wash it carefully in order to remove the hydrochloric acid, which adheres to or is enclosed by the precipitate as completely as possible.
The filtration and washing of the methylsilicic acid create considerable difficulties because the precipitate is very voluminous and thus difficult to filter. Moreover, it occludes considerable amounts of hydrochloric acid within its bulk and some of this acid is inaccessible to the washing process.
In the subsequent reaction with alkali solution, the desired alkali methylsiliconates as well as alkali chlorides are formed, the latter being produced in an amount corresponding to the amount of occluded hydrochloric acid. Because the precipitated monomethylsilicic acid also occludes considerable quantities of water, it is necessary to concentrate the alkali siliconate solutions by distillation if higher concentrations are required.
A further, particularly serious disadvantage of this process is the fact that methylsilicic acid is not stable and condenses to form higher molecular weight products on storage. These materials are either insoluble or only slightly soluble in aqueous solutions of alkali hydroxide.
German Pat. No. 11 76 137 disclosed a process for the preparation of alkali methylsiliconates or their aqueous solutions in which the siliconates are obtained with significantly decreased amounts of contaminating alkali chloride. The process is characterized by the fact that the methylsilicic acid is precipitated from the alkali chloride-containing alkali methylsiliconate solutions by the addition of acid or of acid-releasing compounds until a pH of about 4 is reached below 60.degree. C. Additionally, the separated methylsilicic acid is dissolved in an equimolar amount of alkali solution and, if necessary, the solution is freed from water.
However, the problems arising from the instability of the precipitated methylsilicic acid on storage and the consequence that it must be processed at once, still exist with the process of German Pat. No. 11 76 137. Moreover, according to column 4, lines 6 ff. of this patent, the methylsilicic acid, precipitated according to this process, also contains considerable quantities of water. Thus, even if only a 30% solution is to be prepared from the solution of sodium methylsiliconate which has been formed, water must be removed by distillation.